.beta.-cells are specialized cells that secrete insulin and are found in pancreatic islets. Insulin belongs to a group of protein/polypeptide hormones. Insulin increases the rate of synthesis of glycogen, fatty acids, and proteins and stimulates glycolysis and cell proliferation. It also promotes the transport of glucose, and some other sugars, and amino acids into muscle and fat cells. Insulin levels are regulated to maintain glycemic homeostasis, and an important mechanism for regulating insulin production, and hence insulin levels, is .beta.-cell mass.
During the lifetime of an individual metabolic needs can change drastically, requiring dynamic changes in cells and tissues that regulate homeostasis. During pregnancy (Marynissen et al., Diabetes 36:883-891, 1987) .beta.-cell mass increases, as well as in response to obesity (Kloppel et al., Surv. Synth. Pathol. Res. 4:110-125, 1985). These increases in .beta.-cell mass are attributed to an increased requirement for insulin to maintain normal glucose levels (Parsons et al., Endocrinology 130:1459-1466, 1992). It has also been shown that .beta.-cell mass normally decreases post-partum, primarily by apoptosis (Scaglia et al., Endocrinology 136:5461-5468, 1995).
It is generally believed that increases in .beta.-cell mass occurs in three ways: 1) an increase in cell size and function; 2) increased proliferation of mature .beta.-cells; and/or 3) increased recruitment and differentiation of .beta.-cell progenitors. In diabetic mice, animals that received islet transplants and then achieved normal glycemia, showed .beta.-cell hypertrophy, rather than an increase in cell replication (Montana et al., J. Clin. Invest. 91:780-787, 1993). Adult .beta.-cell regeneration has been demonstrated in rodents (Hellerstrom et al., in "The Pathology of the Endocrine Pancreas in Diabetes", P. J. Lefebvre and D. G. Pipeleers, eds., pp. 141-170, Springer-Verlag, Heidelberg, 1988). In partially pancreatectomized rats both preexisting .beta.-cells, as well as proliferation and differentiation of precursor cells, have been demonstrated to expand (Bonner-Weir, Diabetes Nutr. Res. 5, Supp.1:21-25, 1992).
Several factors have been shown to increase .beta.-cell mass. These factors include glucose (Woerner, Anal. Rev. 71:33-57, 1938), IGF-I (Rabinovitch et al., Diabetes 31:160-164, 1982), reg protein (Terazono et al., J. Biol. Chem. 263:2111, 1988) and possibly a combination of TGF-.alpha. and gastrin (Bonner-Weir, Recent Prog. Hormone Res. 49:91-104, 1994). While some factors have been shown to increase .beta.-cell mass in vitro or in vivo, understanding of the process is poorly understood and the possibility that other unidentified factors are involved is likely.
Recently a new member of the insulin superfamily has been identified, early placenta insulin-like factor or placentin (Chassin et al., Genomics 29:465-470, 1995). Placentin cDNA was isolated from first trimester human placenta and found to have a 139-amino acid open reading frame. Based on homology to the rest of the insulin superfamily it was predicted that placentin, like preprorelaxin and preproinsulin, would have a signal sequence, followed by the B chain, C peptide, A chain. The mature molecule would have the signal peptide and C peptide removed, with the B and A chains joined by both inter- and intra-chain disulfide bonds (Chassin et al., 1995, ibid. and James et al., Nature 267:544-546, 1977). The B-chain, C-peptide, A-chain motif is found in several other proteins, including relaxin (U.S. Pat. No. 4,835,251), insulin-like growth factors (IGF) I and II (Bang and Hall, in "Insulin-like Growth Factors", P. N. Schofield (ed.), pp. 151-177, Oxford University Press, Oxford, 1992), and Leydig Factor (Bullesbach et al., J. Biol. Chem. 270:16011-16015, 1995). Unlike other members of the insulin superfamily, IGF I and IGF II have D and E domains that are cleaved post-translationally. Cysteines that are involved in disulfide bonds are conserved in all the members of the family and play a role in the tertiary structure of the molecules.
Placentin has been shown to stimulate .sup.3 H-thymidine uptake in human placental 3AsubE cells and stimulate human chorionic gonadotropin production in primary cultures of trophoblasts (Koman et al., J. Biol. Chem. 271:20238-20241, 1996). This activity suggests that placentin may play a role during placental development. However, the present inventors, surprisingly, have found that a molecule encoded by the DNA for placentin, but a different amino acid structure, increases .beta.-cell mass and may be useful in treatment of diabetes, and further that the biologically active molecule differs from the molecule described in the art.